1. Field of the Invention
The present invention relates to a method for calibrating the sensitivity of an acceleration sensor device capable of detecting at least one of components, which respectively correspond to the directions of three orthogonal axes, namely, X-axis, Y-axis, and Z-axis, of an externally acting acceleration, which acts thereon, by using an acceleration detecting device consisting of a pair of piezoelectric elements.
2. Description of Related Art
In the automobile and machine industries, there has been an increase in demand for sensors capable of accurately detecting physical quantities, such as force, acceleration, and magnetism. Especially, compact sensors capable of detecting each of two-dimensional or three-dimensional components of such physical quantities are demanded.
For example, a sensor having a plurality of piezoelectric elements mounted on a flexible plate mounted on a flexible substrate having an operating member is disclosed in the Japanese Unexamined Patent Publication No. 5-26744.
This sensor is configured so that the flexible substrate deforms according to a physical quantity externally acting on the operating member. The direction and magnitude of the externally acting physical quantity are detected by a single sensor device in a three-dimensional manner on the basis of electric charges that are produced in the piezoelectric elements according to the strain due to the deformation of the flexible substrate.
This will be explained hereinbelow by taking an acceleration sensor, which has an operating member as a weight, as an example of such a sensor device. In the case that an externally acting acceleration a is exerted on the sensor device as illustrated in FIGS. 2A and 2B, an inertial force f acts on the weight 10 in a direction opposite to the direction of the acceleration a. This inertial force f causes the deformation of flexible substrate 12 put on the weight 10 and supports 11.
Electric charges are produced in the piezoelectric materials 13 according to the direction and magnitude of strain due to the deformation and according to the polarization direction and magnitude of the piezoelectric materials 13 put on the flexible substrate 12. Thus, the detection of the direction and magnitude of the externally acting acceleration is enabled by outputting the electric charges from upper electrodes 17x, 17y, and 17z, and a lower electrode 18 as electric signals.
The aforementioned acceleration sensor device is configured so that components of the externally acting acceleration, which respectively correspond to the directions of X-axis, Y-axis, and Z-axis, are detected by a single sensor device as components. As a result, for example, even when the weight 10 undergoes the influence of the acceleration only in the direction of Z-axis, as illustrated in FIG. 3B, the strain due to the deformation occurs not only in the piezoelectric element 14x for detecting X-axis component of the acceleration, but in the piezoelectric element 14y (not shown in FIG. 3B) for detecting Y-axis component of the acceleration. Consequently, electric charges are produced in the piezoelectric elements 14x and 14y. 
The weight 10, however, does not undergo the influence of the acceleration only in the directions of X-axis and Y-axis, so that it is necessary to prevent electric outputs of the electric charges produced in the piezoelectric elements 14x and 14y from being electrically outputted therefrom.
Thus, the aforementioned acceleration sensor device employs a method of electrically canceling the produced charges by configuring the pair of piezoelectric elements as a single acceleration detecting device.
Practically, as illustrated in FIGS. 2A and 2B, an acceleration detecting device, which corresponds to each of X-axis, Y-axis, and Z-axis, of the acceleration sensor device comprises at least one pair of piezoelectric elements placed at positions that are symmetric with respect to the weight 10.
Because of the symmetric positions of the pair of piezoelectric elements with respect to the weight 10, the amounts of strain of these piezoelectric elements of the pair are almost equal to each other.
Polarization of the same magnitude is performed on the piezoelectric elements so that, among piezoelectric materials constituting the piezoelectric elements of the pairs, the piezoelectric materials to be used for detecting X-axis component and Y-axis component of the acceleration have opposite polarities, and that the piezoelectric materials to be used for detecting Z-axis component of the acceleration have the same polarity. Therefore, when the weight 10 is oscillated in the direction of Z-axis as illustrated in FIG. 3B, the electric charges of opposite polarities produced in the piezoelectric elements for detecting X-axis component and those (not shown) for detecting Y-axis component are canceled. Thus, no electric signals are outputted from these piezoelectric elements. On the other hand, when the weight 10 is oscillated in the directions of X-axis or Y-axis as illustrated in FIG. 3C, the electric charges produced in the piezoelectric elements 14z for detecting Z-axis component are canceled, so that no electric signals are outputted from these elements 14z. 
However, sometimes, the quantities of electric charges to be produced in the piezoelectric elements of the pair are not equal to each other owing to defective conditions at the time of forming the piezoelectric elements, for instance, variation in the electrode area of the piezoelectric elements, variation in the dielectric constant of the piezoelectric elements, a deviation of the position of the weight, and variation in deformation caused by the strain of the flexible substrate.
In such a case, the electric charges produced in the piezoelectric elements of the pair are not completely canceled but outputted therefrom as electrical signals. Thus, for example, the sensitivity in the direction of X-axis is indicated despite the fact that the acceleration sensor device undergoes the influence of the acceleration only in the direction of Z-axis (hereunder, such sensitivity will be referred to as xe2x80x9cnoise sensitivityxe2x80x9d).
It is necessary for ensuring the reliability of the sensor to limit a ratio of the noise sensitivity to the sensitivity in the direction of an axis to be detected (hereunder, such sensitivity will be referred to as xe2x80x9cprincipal axis sensitivityxe2x80x9d) within a predetermined range (for instance, if the principal axis sensitivity is 100%, the noise sensitivity should be equal to or less than 5%). On the other hand, it is very difficult to limit the noise sensitivity within the predetermined range in the process of manufacturing the acceleration sensor devices. Thus, there is the necessity for a method for calibrating the sensitivity of the sensor device after manufactured.
The present invention is accomplished in view of the aforementioned circumstances.
Accordingly, an object of the present invention is to provide a method for calibrating the sensitivity of an acceleration sensor device, according to which electrical outputs of piezoelectric elements of a pair of an acceleration detecting device are canceled and the aforementioned noise sensitivity is suppressed.
To achieve the foregoing object, according to the present invention, there is provided a method for calibrating the sensitivity of an acceleration sensor device capable of detecting an externally acting acceleration by at least one acceleration detecting device comprising a pair of piezoelectric elements. This method includes the steps of applying oscillation to said acceleration sensor device, and applying a voltage of a polarity, which is opposite to the polarity of polarization being already present therein, to one of the piezoelectric elements of the pair, whose electric output has an absolute value larger than an absolute value of an electric output of the other piezoelectric element of the pair. Thus, the absolute value of the electric output of one of the piezoelectric elements of the pair is brought close to that of the electric output of the other piezoelectric element of the pair.
According to the method for calibrating the sensitivity of an acceleration sensor device of the present invention, an acceleration sensor device comprising at least one acceleration detecting device may be employed as an acceleration sensor device having the sensitivity to be calibrated. This acceleration detecting device comprises a weight, supports each installed around the weight in such a way as to have a hollow portion, a flexible substrate put on the supports so that the weight is suspended in the hollow portion of each of the supports, and a pair of piezoelectric elements.
Further, according to the method for calibrating the sensitivity of an acceleration sensor device of the present invention, an acceleration sensor device comprising acceleration detecting devices respectively corresponding to two orthogonal axes among orthogonal X-axis, Y-axis, and Z-axis may be employed as an acceleration sensor device having sensitivity to be calibrated. Alternatively, according to the method for calibrating the sensitivity of an acceleration sensor device of the present invention, an acceleration sensor device comprising acceleration detecting devices respectively corresponding to three orthogonal X-axis, Y-axis, and Z-axis is employed as an acceleration sensor device having sensitivity to be calibrated.
The method for calibrating the sensitivity of an acceleration sensor device of the present invention, preferably, further comprises the steps of preliminarily calculating the relation among the value of the voltage, the time period, during which the voltage is applied to the piezoelectric element, and the magnitude of polarization changed by applying the voltage thereto when the voltage of the polarity opposite to the polarity of the polarization being already present therein, and determining the value of the voltage to be applied thereto, and the time period, during which the voltage is applied thereto.
Furthermore, the method for calibrating the sensitivity of an acceleration sensor device of the present invention may further comprise the step of simultaneously applying the voltage of the polarity, which is opposite to the polarity of the polarization being already present therein, to the piezoelectric element, whose electric output has an absolute value larger than an absolute value of an electric output of the other piezoelectric element of each of the pairs, and applying a voltage of the same polarity as the polarity of the polarization, which is already present, to the piezoelectric element, whose electric output has a smaller absolute value, of a corresponding one of the pairs by energizing all of the acceleration detecting devices.